1. Field of the Invention
This invention relates to piezoelectric materials and especially relates to 3-1 and 3-2 phase connected composites of piezoelectric ceramics and polymers for use as pressure, sound, and vibration sensors and electro-mechanical transducers. It especially relates to the fabrication by molding of intricate, functional shapes of such composites with uniform and reproducible density and the assembly of composite elements into precision arrays.
2. Review of the Prior Art
A variety of electro-mechanical transducers such as hydrophones, air sensors, vibration sensors, pressure and stress sensors depend on the piezoelectric phenomenon exhibited by certain piezoelectric crystals, polarized ceramics, and polarized polymers.
An important class of prior art sensors have as their active sensing element solid shapes of piezoelectric ceramic materials. In the particularly important hydrophone application area, the piezoelectrically active ceramic component converts underwater sound pressure waves to electrical signals which are then ampliiied and displayed. In recent years large numbers of sensors have come to be used in wide aperture arrays of individual sensors.
The solid shapes are fabricated from piezoelectric ceramic powders by a variety of standard consolidation techniques. Electrodes are bonded to two opposing faces in order to polarize the piezoelectric material during fabrication and to sense the electrical signals that develop in the material in use. Lead zirconate titanate compositions, which are collectively referred to as PZT materials, are widely used because transducers based on these materials exhibit moderate sensitivity and durability. Solid shapes in the forms of cubes, plates, tubes, and arrays thereof are commonly employed.
The detecting sensitivity of such prior art dense solid piezoelectric ceramics (piezo-ceramic) sensors, however, is modest. The sensitivity of a sound receiver material is commonly characterized by a hydrophone Figure of Merit or FOM. The FOM is defined as the product of two hydrostatic piezoelectric coefficients--the charge coefficient, d.sub.h, and the voltage coefficient, g.sub.h. For conventional PZT-based piezo-ceramics d.sub.h typically has values of about 50-60.times.10.sup.-12 Coulombs/Newton (C/N) and g.sub.h has values of about 2-3.times.10.sup.-3 volt.times.meter/Newton (Vm/N) to give FOM values in the 100-200.times.10.sup.-15 m.sup.2 /Newton (m.sup.2 /N) range.
A more sensitive hydrophone element is described in U.S. Pat. No. 4,422,003 wherein PZT shapes (cubelets) are fabricated having holes in the PZT parallel to, and between, the electroded surfaces. The holes are filled with a liquid polymer resin which is solidified in place. The result is described as a piezo-ceramic polymer composite with 3-1 or 3-2 connectivity. In this nomenclature for the connectedness of biphasic composites, "3" refers to the fact that the ceramic phase is self-connected along all three cartesian coordinates of the specimen, while "1" and "2" refer to self-connectivity of the polymer phase along one or two coordinates, respectively.
According to this disclosure, biphasic composites with 3-1 and 3-2 connectivity exhibit relatively high hydrostatic piezoelectric voltage (g.sub.h) coefficient and hydrophone FOM values. FOM values over 10,000.times.10.sup.-15 m.sup.2 /N are observed. The presence of the 3-1 and 3-2 connectivities is chiefly responsible for the improved hydrophone FOM values.
In addition, other piezoelectric voltage coefficients, such as g.sub.33, are much higher from composites than from solid PZT elements.
Still further, the density of these composites is lower (4-5 g/cm.sup.3) than that of solid PZT (ca. 8 g/cm.sup.3). The lower density of the composites results in a better acoustical coupling of the transducer to the sound-transmitting water phase.
The most successful 3-2 shape described in this patent is a cubelet, of approximately 9/32 inch along each edge, with two circular tunnels, approximately 5/32 inch in diameter, joining the opposite faces. The tunnels contain a high compliance solid polymer, such as epoxy, bonded well to the ceramic walls. A typical 3-1 composite, which has a lower FOM, is a similar cubelet with only one resin-filled hole or a parallel plurality thereof.
The PZT-polymer composites are prepared according to this patent by conventional means, including cold pressing, sintering, manually drilling out the transverse cores ultrasonically, and filling them with a liquid epoxy or other curable resin. Such a fabrication process is slow and costly because most sintered specimens fracture during the ultrasonic drilling of the second core or row of cores. Consequently, it is unsuitable for the production of a large number of such composites which must be dimensionally identical and free of defects in order to be assembled into advanced multisensor-containing arrays. There is accordingly a need for a simple and easily reproducible process for making the PZT-polymer composites in a reproducible and rapid manner without fracturing thereof during manufacture.
U.S. Pat. No. 738,423 discloses a method for casting cored pieces such as building blocks by utilizing a mold with removable cores. Openings are provided in the flask for coring in two directions, but cross-coring, i.e., molding two intersecting through holes into the specimen transverse to the pressing axis, which provides the highest FOM values in a piezoelectric material, is not considered.
U.S. Pat. No. 2,305,877 describes a method for producing spark plug insulators, wherein a ceramic mateial is molded by utilizing a plunger and a needle. The injected ceramic material surrounds the needle within the mold so that the needle simultaneously produces one central bore and the thread in the head of the insulator.
U.S. Pat. No. 2,919,483 discloses a method for forming a ceramic capacitor having a plurality of longitudinally extending holes in the form of thin slices therein. The method utilizes a pool of mercury in a tank having a dividing wall that extends beneath the mercury surface, a liquid coagulating agent being floated on the mercury on one side of the dividing wall and an aqueous dispersion of ceramic raw material being floated on the other side of the dividing wall. The process comprises passing a plurality of unrefractory thin strips successively through the coagulating agent, the mercury, and the aqueous ceramic dispersion, compressing the strips together, heating the compressed strips to remove much of the moisture from a newly formed green ceramic body, and firing the resulting structure to eliminate the unrefractory strips, which may be parchment paper about 0.003 inch thick and 1/4 inch wide.
U.S. Pat. No. 3,830,458 describes a mold for the casting of concrete articles which may be easily opened and then accurately reassembled without being damaged by relative movement with the mold sides.
U.S. Pat. No. 4,023,769 discloses a method for molding concrete slabs which utilizes opposed core members during the molding operation. The core members do not meet, thereby producing a concrete slab having a web rather than core. If it is desired to form communicating openings in the central web of the slab, additional core members may be provided on the inner end or ends of the core members which may be of any desired shape or size, thereby forming round openings for pipes or long narrow openings for heating or air conditioning ducts. These concrete slabs are used to form large building slabs suitable for either walls or floors, having longitudinal transverse webs of great structural strength and yet capable of being molded substantially to any desired size, thickness, and length.
U.S. Pat. No. 4,402,894 describes a method for molding a ceramic resistor utilizing a circular mold with vertically opposed plungers, compaction being provided only by the upper plunger, and the lower plunger being used only for ejecting the molded article, such as a ceramic resistor blank. The method particularly discloses the use of a stationary resilient finger and means mounting the finger so that a sharp rotational movement is imparted to the plunger relative to the pressed blank as the applied pressure thereon is being released, thereby providing a shearing action which insures a clean separation of the blank from the plunger.
In the Final Report to the Office of Naval Research on Contract No. N00014-82-G-0072 TaskII, entitled "Hinged Hydrostatic Transducers", a method of arranging a plurality of composite cubelets in an array to increase the sensing area of a transducer and its sensitivity is discussed and evaluated. The cubelets were manually arranged in the desired array and set in place with a curable resin such as a polyurethane or epoxy resin. To precisely arrange large numbers of cubelets requires skill and attention and is therefore only practical when small numbers of arrays are required.
Fixing the cubelets in place by setting the array in a resin entails working with resins, curing agents, and curing cycles and requires special skills which are not necessarily available in an assembly line environment.
Finally, the rigidity of the potting material of the array affects the performance of the array as a hydrophone. Highest FOM's result when the holes in the cubelets are sealed from the environment by stiff plates. When more compliant sealing materials are used, the transverse hydrostatic pressures are converted in accordance with Poisson's ratio effects to longitudinal stresses in the cubelet which oppose the pressure being measured. The result is a hydrophone that is less sensitive and which has a response that is static pressure dependant.
For these reasons the prior art approach is only practical when a few arrays are to be assembled and where high sensitivity and static pressure limitations are not a concern.
A means must be provided to simplify the process.